:
: But the Shorter Oxford says the first recorded use of "fluke" in the sense "a successful stroke made by accident or chance" is in 1857, and suggests that it derives either from a dialect word meaning "guess/miss in fishing" or is a pun on another meaning of "fluke", viz. "flounder" (the flatfish) - in other words, if you make a fluke, you're just floundering and your success is just due to luck. (VSD)

Entirely possible. Fluke (the fish) is easy to catch. A bottom feeder. When you catch one, it flops around seemingly randomly.

Perhaps fluke (the accident) is related to one or more of those characteristics: gullibility, or randomness. We have the example of "mullet" in more recent times. This fish is famously stupid and easy to catch, and stockbrokers (for at least 40 years that I know) have used the term for those customers who will buy almost anything recommended. No offense to any dentists out there, but financial advisors do regard your profession as mullet-y. Later, a haircut (long on the back, short on the sides) became known as a Mullet, for reasons that need no elaboration here.